1. Field of the Invention
The present invention relates to an erasable ink, and more particularly, to an erasable ink containing a thermoplastic block copolymer.
2. Description of the Prior Art
In choosing a writing instrument, line intensity and erasability must be taken into account. Until recently, the choice was between these two factors since it was impossible to have both high line intensity and high erasability in the same writing instrument. Writing instruments such as pencils possess high erasability (i.e., the written line can be removed readily) but are relatively low in line intensity (i.e., the contrast between the written line and the paper is low). Because of this, a xerographic copy of a document written in pencil is typically a poor reproduction. In contrast, writing instruments such as ball point pens, fountain pens, and soft- or porous-type pens produce written lines which are relatively intense but lack any significant degree of erasability without damage to the underlying writing surface, or require additional material to mask the unwanted lines.
Accordingly, the prior art contains numerous attempts at writing instruments containing inks which produce a written line of high intensity, and yet are erasable for at least the first few hours after being placed upon the writing surface. This combination produces a written document having all the desirable attributes of both systems, and within a short period of time after being written, possess the permanency typically desired in written documents.
One such system is described in U.S. Pat. No. 3,875,105 to Daugherty et al., issued on Apr. 1, 1975. That patent describes an erasable writing medium suitable for use in ball point pens having a discontinuous phase of a solid colorant and a homogeneous continuous phase including a matrix material having cohesive properties exceeding its property to adhere to the substratum being written upon. The ink consists of a pigment, stabilized with adsorbed surfactant, dispersed in a multicomponent solvent system containing a polyvinyl methyl ether elastomer. On transfer to paper, the low boiling toluene evaporates into the air or penetrates into the paper causing a viscosity increase and enhancement of the elastic characteristics of the pigment dispersion. Adhesion between the elastomer and pigment remains greater than that between the elastomer and paper until certain components of the solvent system migrate into the paper, at which time polymer adhesion increases and erasability decreases. The erasability of the ink is stated to depend in part on the rapid evaporation of at least one of the components of the continuous phase. Accordingly, the volatile component is stated to be one preferably with an evaporation rate of 3 to 15 on a relative numerical scale on which ethyl ether is assigned an evaporation rate of 1, such as toluene which is stated to have a rate of 6.1. The volatile component is employed in a range from 24.0 to 29.0%. The line intensity of that ink is dependent upon the relative quantity of pigment used. As stated in that patent, it may be necessary to dispense the ink in a pressurized ball pen depending upon the relative quantity of pigment used since at higher levels of pigment the ink may not flow with sufficient rapidity through the feed passageways of the conventional, gravity-fed ballpoint pen. Moreover, in an ink of such a high level of volatile component, some means must be provided to prevent the volatile component from evaporating into the atmosphere which would increase the viscosity of the ink while in the pen.
U.S. Pat. No. 4,097,290 to Muller et al., incorporated herein by reference, describes a ballpoint writing instrument capable of writing with an intense line which is easily erasable by mechanical means for an initial period of several hours but eventually becomes non-erasable. These properties are attained by an ink composition containing certain specific rubbers and volatile solvents which control erasability. The rubbers employed are natural or of a chemical structure essentially duplicating that of natural rubber. In addition, the ink contains a volatile low boiling organic solvent having a boiling point less than 108.degree. C. and exhibiting 100% evaporation within 60 minutes, and a high boiling organic liquid solvent having a boiling point of greater than 300.degree. C. The volatile low boiling organic solvent is included to rapidly increase the viscosity of the ink in the written line thereby minimizing penetration by the ink into the paper. Although the ink yields a very intense line with good erasability for the few hours after it is written, it suffers from the same disadvantage as that set forth previously for the Daugherty et al. Patent. Namely, it possesses a relatively high concentration of a low boiling, rapidly vaporizable solvent, and a relatively high concentration of pigment such that means for subjecting the ink to superatmospheric pressure is necessary to facilitate the flow and supply of ink to the ball. In addition, the rubber must be milled prior to its incorporation into the ink to produce an average molecular weight of between 100,000 and 900,000 and preferably between 400,000 and 750,000. The milling process is somewhat difficult to control in that milling too rapidly, or for too long a time, will produce rubber particles unsuitable for use in the ink.
There are often also problems in addition to time and expense when ink must be supplied under pressure. If the ball doesn't fit almost perfectly in the ball seat, point bleed can occur, and will become worse as the ball seat wears with writing. Writing smoothness is also affected by pressurization because the ball must be pushed into its seat, and the ink pressure must be counteracted in order to get ink flow. Another major disadvantage of prior art erasable ink is that the written line is easily smeared.
Accordingly, a need exists for an erasable ink for use in ballpoint pens which will produce a line of high erasability and high line intensity, which does not depend upon a high level of volatile solvent, does not smear, is readily prepared without the necessity of controlled milling of the rubber component, and requires little or no pressurization.